The megathrombocyte has been shown, by our laboratory to be a large-heavy platelet released 'early' from the bone marrow, and therefore designated young. Its life span in the peripheral circulation would be examined to determine whether it: 1) Breaks up into smaller platelets during its life-span; 2) Retains its size throughout a normal platelet life-span; 3) Retains its size throughout a shortened platelet life-span. Megathrombocytes have also been shown to be functionally more active than other platelets. Procedures would be adapted for their atraumatic isolation for purposes of platelet transfusion. We have identified a splenic-dependent thrombopoietic humoral factor from plasma of chronically-bled animals with thrombocytosis. Experiments would be designed to test the hypothesis that this represents a 'splenic release' factor. We have shown that Fe compounds are required for platelet production and protein synthesis. Iron is also required for granulocyte and lymphocyte protein synthesis. The possible role of iron as a universal regulator of protein synthesis would be examined. Autoimmune thrombocytopenic purpura has been studied extensively in our laboratory with a PF-3 anti-platelet antibody test. Attempts will now be made to characterize the antigenic specificity of anti-platelet antibody; and to develop a radio-immune assay for this purpose. We have recently discovered a qualitative platelet defect associated with anti- platelet antibody and increased megathrombocytes. Experiments would be designed to determine whether these patients have an 'anti-platelet function' factor.